Molecular motions in threadlike micelles

Molecular motions of a cetyltrimethylammonium cation (CTA(+)) and a salicylate anion (Sal(-)) in threadlike micelles formed by themselves in aqueous solution were examined by use of fluorescence probe techniques. Long and stable threadlike micelles are formed around the equimolar ratio of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal), and the system shows a profound viscoelasticity. Cetylacridinium orange bromide (CAOB) and sodium 2-hydroxy-3-naphthoate (NaHNA) were employed as substitute fluorescence probes for the CTA(+) cation and the Sal(-) anion, respectively. The fluorescence lifetime and the steady-state fluorescence anisotropy of these probes were measured with varying the NaSal concentration, while the concentrations of CTAB and the probes were kept constant. Since the probes had unimodal fluorescence lifetimes at any molar ratios of NaSal to CTAB, the conventional Perrin equation relating the fluorescence lifetime and rotational relaxation time (tau(phi)) through fluorescence anisotropy was used to evaluate tau(phi) of the probes. The tau(phi) values for both the probes are essentially independent of the molar ratio of NaSal/CTAB, once the threadlike micelle has grown sufficiently above a ratio higher than unity. These imply that molecular motions of the probes, which may be identical to those of the CTA(+) cation and the Sal(-) anion in the threadlike micelle, are essentially independent of macroscopic viscoelastic behavior of the micellar system such as the longest relaxation time or viscosities; Because the transition moment of the probe CAO(+) cation is located within the 9-carbon to 10-nitrogen direction of an acridinium head group, the lateral diffusional coefficient of the CAO(+) cation, which should be quite similar to that of CAT(+), in the threadlike micelle is roughly evaluated to be ca. 1 x 10(-5) cm(2) s(-1) with a simple equation of D-lat similar to a(2)/tau(phi), where a is the radius of the micelle.